(1) Field of the Invention
The present invention relates to a ring-shaped data communication network in which plural slave stations and a master station are serially connected in a closed loop via transmission lines.
(2) Description of the Background Art
For data communication applications, a closed loop communication system often is used to transfer information in the form of data strings composed of serial data, via a transmission line, between a master station and a plurality of slave stations.
FIG. 5 illustrates the general configuration of this type of data communication system. Referring to FIG. 5, a master station 1 and several connection boxes 2A-2D are connected into a ring-shaped network by a plurality of transmission cables 4A-4E, acting as data transmission lines connecting adjacent ones of the master station 1 and the connection boxes 2A-2D in series. Portable data communication terminals 3A, 3C and 3D serve as slave stations, which are removably connected to the connection boxes 2A, 2C and 2D, respectively. A data communication apparatus is comprised of one slave station and its connection box.
FIG. 6 is a block diagram illustrating the configuration of a data communication apparatus which is known in the art and is comprised of the connection box 2A and the slave station 3A shown in FIG. 5. In FIG. 6, switching device 6 is incorporated in the connection box 2A for switching from one data transmission line to another by moving a contact 6A from position A to position B when a coil 6B is excited. A communication interface circuit is incorporated in the slave station 3A for transferring data to and from the master station 1 via the ring-shaped network. Each data communication apparatus has several terminals 10A-10H for connecting corresponding signals with the connection box 2A. Jumper 8 is connected across terminals 10D and 10E of the connection box 2A for purposes that will be defined later. Power is fed from supply 9, located outside box 2A, for driving the switching device 6.
FIG. 7 shows the transmission frames of a data string sent by the master station 1. The data string is led by a flag 60 representing data transmission "start". There are "n" bits per station such that the data string for "m" stations comprises "n x m"-bits of data 61 plus an idle space 62 wherein there is no information transmission. The size of the idle space is selected for a given transmission rate to provide a sufficient duration for processing of the transmitted data at a receiving station.
FIG. 8 illustrates the details of the transmission frame in FIG. 7, wherein the flag 60 and the data 61 together comprise a serial data string composed of "1"s and "0"s, and the idle space 62, indicating a no-communication state, is represented by a succession of "0"s. In particular, the data string is divided into a flag waveform 60A composed of a flag sequence such as "111101", a data waveform 61A composed of trapezoidal pulses, and an idle waveform 62A having a succession of "0"s. The idle period generally encompasses at least 4 bits, representing a minimum duration of 1 ms.
Operation of the prior art data communication apparatus will now be described with reference to FIGS. 6 to 8. Referring to FIG. 6, when the slave station 3A is not connected with the connection box 2A, the terminals 10D and 10E of the connection box 2A are disconnected from each other. Since the terminals 10D and 10E are disconnected from each other in relation to the switching device 6, the voltage of the switching device driving power supply 9 is not applied to the switching device 6 and the contact 6A of the switching device 6 is in position A. Hence, the data transmission line 4A is linked to the data transmission line 4B to the next station in the ring-type network, via the contact 6A of the switching device 6. That is, the connection box 2A bypasses the slave station 3A and the data string transmitted by the master station 1 is transmitted intact to the next station.
When the slave station 3A is connected with the connection box 2A, the terminals 10D and 10E are connected to each other by the jumper 8 in the slave station 3A, voltage is applied from the switching device driving power supply 9 to the switching device 6, and the contact 6A of the switching device 6 is changed from position A to position B. When the contact 6A is switched from position A to position B, the connection box 2A is set to a slave station insert state, the data receive terminal of the slave station 3A is connected with the data transmission line 4A from the preceding station, and the slave station 3A is then ready to receive data from the master station 1.
The data transmission frames are sent as a data string by the master station 1 by reading data from a data "frame" relevant to itself and/or writing its own data to the frame. The data no-communication state, referred to as the "idle" space 62, exists behind the final communication frame, i.e., after the (flag)+(n x m bits). The space 62 has a succession of four or more "0"s and continues for one or more ms.
A problem arises when the slave station is connected with the connection box and the data transmission line switches from the slave station bypass state to the slave station insert state, without regard for the presence on the data transmission line of a data string transmitted by the master station. In this case, the data string is likely to be interrupted. When the succeeding slave stations and the master station receive the interrupted data string, a communication error will result.
It is accordingly an object of the present invention to overcome the disadvantages in the prior art by providing a data communication apparatus which can switch a data transmission line from a slave station bypass state to a slave station insert state without interrupting a data string transmitted by a master station when a slave station is connected with a connection box.
It is a further object of the present invention to provide a data communication apparatus that will switch a data transmission line from a slave station bypass state to a slave station insert state upon detection of a predetermined signal pattern in a data string transmitted by a master station.
It is yet another object of the present invention to provide a data communication apparatus at each of a plurality of stations on a ring type network that will switch a data transmission line from a slave station bypass state to a slave station insert state at the same relative time during reception of a data string transmitted by a master station.